This invention is related to the field of solid state devices and, more particularly, to solid state optical-electronic system.
Solid state optical-electronic systems are useful in a wide variety of applications. One area, for example, where such equipment has become increasingly important is in optical communications systems, in which information is transmitted by means of a signal in the optical range of the electromagnetic spectrum. The uses and requirements of solid state optical-electronic systems in the optical communications environment will serve to exemplify the importance of such technology.
The availability of optical fibers which exhibit a very low loss and near zero dispersion has made feasible the development of optical communication systems capable of operating at very high data rates. Three other major components, however, must be provided before a practical high data rate communications system may be realized. Required are a reliable, high speed light source, a high speed, high quantum efficiency photodetector, and very low noise, high speed electronics which can be applied to signal processing and control functions.
The development of high performance photodetectors, such as avalanche photodetectors (APDs), has provided the necessary high quantum efficiency and has led to important advances in fiber optics communications systems, as well as in other areas, such as active laser imaging, laser satellite communication, laser diagnostics, laser range finding, and picosecond light pulse measurements. The application of III-V alloys to photodetector technology has further led to the development of avalanche photodetectors which exhibit superior properties in comparison to previous designs utilizing germanium and silicon. By adjusting the alloy composition, for example, the wavelength response of a III-V alloy photodiode can be tuned. Moreover, because of the high absorption coefficients of the direct bandgap III-V alloys, the quantum efficiency of such a photodiode is high, even when the diode is fabricated with a narrow depletion region to provide a high speed response. In addition, heterostructure window layers can be grown in III-V alloy photodiodes so that high speed performance is obtained while at the same time the loss of photogenerated carries due to surface recombination is reduced.
Another component which is required in an optical communications system is low noise, high speed processing electronics, which has been supplied with the development of the transimpedance amplifier. By hybrid integrating a high speed, high quantum efficiency III-V avalanche photodetector with a transimpedance amplifier utilizing GaAs FET electronics, an optical receiver has been produced with from 10 to 20 times better sensitivity than is available with germanium APDs. Such hybrid receiver designs, however, introduce construction difficulties due to the need to interface the optical device with the necessary control and signal processing circuitry and achieve a system with a desired level of performance. It would be advantageous to integrate these optical and electronics functions in a single unit, but this has not heretofore been possible with traditional silicon-based devices, because there are no semi-insulating substrates available in silicon technology to provide the necessary isolation between the electronics and the detector. Furthermore, the optical properties of silicon necessitate an optically active layer of considerably greater thickness (10-100 .mu.m) than can be easily fabricated by available techniques. A practical integrated optical receiver design, however, would be an advantageous addition to optical technology and would be particularly useful in the area of optical communications systems.